Q135913: FIX: Bad POINT Structure in MSG Returned by GetCurrentMessage

Article: Q135913
Product(s): Microsoft C Compiler
Version(s): winnt:
Operating System(s): 
Keyword(s): kbDocView kbMFC kbVC100bug kbVC150bug kbVC200bug kbVC200fix kbVC210bug kbVC220fix kbWnd
Last Modified: 06-MAY-2001

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The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++ 32-bit Edition, versions 2.0, 2.1 
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SYMPTOMS
========

MFC provides a function called GetCurrentMessage that returns a pointer to a MSG
structure encapsulating the current message being processed. The MSG structure
contains a member variable (pt), of type POINT that contains incorrect values.

CAUSE
=====

The GetCurrentMessage function fills in the POINT structure with the following
line of code:

     *((DWORD*)&pThreadState->m_lastSentMsg.pt) = ::GetMessagePos();

This code was correct in the 16-bit version of Visual C++ because a POINT
structure was made up of two intergers (int) that were each 16-bits. Thus the
size of a POINT was the same as a DWORD.

However, in the 32-bit version of Windows and of the Visual C++ compiler, a POINT
structure is made up of two LONGs, each 32-bits, so the total size of a POINT is
now 64-bits and the above assignment is not valid.

RESOLUTION
==========

To obtain the correct value for the point in the current MSG you can use MFC's
overridden CPoint constructor which takes a DWORD as an argument. For example:

     CPoint pt(::GetMessagePos());
     // Now pt has the correct values

Please note that because the GetCurrentMessage function returns a pointer to a
'const' MSG structure, you can't change the pt member of the message structure.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This problem was corrected in Visual C++ version 2.2.

Additional query words: 2.00 2.10 3.00 3.10

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Keywords          : kbDocView kbMFC kbVC100bug kbVC150bug kbVC200bug kbVC200fix kbVC210bug kbVC220fix kbWndwMsg kbDSupport kbGrpDSMFCATL kbArchitecture kbNoUpdate 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:
Issue type        : kbbug
Solution Type     : kbfix

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